1. Technical Field
The present invention relates to a motor drive apparatus and an electric power steering apparatus using the same, and particularly to a motor drive apparatus capable of detecting an angular velocity of the motor accurately both at the times of high speed rotation and low speed rotation and to an electric power steering apparatus provided with the motor apparatus.
2. Prior Art
In the electric power steering apparatus which provides an automobile steering apparatus with steering assist force with a rotation force of the motor, a drive force of the motor is provided to a steering shaft or a rack shaft with a transmission mechanism such as gears or belt through reduction gears as the steering assist force. A simple structure of such an electric power steering apparatus will be described with reference to FIG. 1. A shaft 102 of a steering handle 101 is connected to a tie rod 106 of steered wheels through reduction gears 103, universal joints 104a, 104b and a pinion rack mechanism 105. The shaft 102 is provided with a torque sensor 107 for detecting a steering torque of the steering handle 101 and a motor 108 for assisting the steering force of the steering handle 101 is connected to the shaft 102 through the reduction gears 103.
The motor 108 of the electric power steering apparatus having such a structure needs to execute control properly so as to output a desired torque corresponding to an operation of the steering handle by a vehicle driver. A vector control, which is one of the typical control methods for controlling the motor 108 of the electric power steering apparatus has been described in Japanese Patent Application Laid-open No.2001-18822 A, and FIG. 2 is a block diagram showing the control apparatus of the electric power steering apparatus disclosed in the patent document.
If explaining the control block, a torque command value Tref calculated by a torque command value calculating section (not shown), a rotation angle θ which is an electric angle of the motor 108 and an angular velocity ω are inputted to a current command value calculating section 204 based on a steering torque Tr detected by the torque sensor 107. Although the current command value calculating section 204 calculates a current command value Iqref of q-axis component and a current command value Idref which is d-axis component, usually, the current command value Iqref is proportional to a torque command value Tref and the current command value Idref is “0” (usually, Idref=0). On the other hand, an angle detector for detecting a rotation angle θ and an angular velocity ω of the motor 108 is provided and although an encoder and a Hall sensor are available as an angle detector, a resolver 201 is used here. Since a signal outputted from the resolver 201 does not indicate digital rotation angle θ, the digital rotation angle θ and the digital angular velocity ω are calculated by a position detecting circuit 202 which is constituted of an RDC (resolver/digital converter) circuit.
This control block diagram uses feedback control as an example and it is necessary to detect actual motor currents Ia, Ib, Ic of the motor 108 for the aforementioned current command values Iqref and Idref and execute feedback control. More specifically, in current detectors 205-1, 205-2, the motor currents Ia and Ic are detected and the motor current Ib is calculated as “Ib=−(Ia+Ic)” by a subtracting section 207-3 from the relation of “Ia+Ib+Ic=0”. Next, for the vector control, it is converted in to motor currents Iq and Id by a 3-phase/2-phase converting section 206. Forth is conversion, the rotation angle θ of the motor 108 is used. Next, the motor currents Iq and Id are respectively fed back to the subtracting sections 207-1 and 207-2 and the subtracting section 207-1 calculates a deviation ΔIq between the current command value Iqref and a motor current Iq and the subtracting section 207-2 calculates a deviation ΔId between the current command value Idref (usually Idref=0) and the motor current Id.
Input is made into a proportional-integral (PI) control section 208 in order to eliminate the above deviations and voltage command values Vdref and Vqref are outputted. Because the actual motor 108 needs to be supplied with 3-phase current, the voltage command values Vdref and Vqref are converted into voltage command values Varef, Vbref, Vcref of three phase by a 2-phase/3-phase converting section 209. A PWM control section 210 generates a PWM control signal based on the voltage command values Varef, Vbref, Vcref and an inverter circuit 211 supplies current to the motor 108 based on the PWM control signal and the motor currents Ia, Ib, Ic are supplied to eliminate the deviations of the current command values Iqref, Idref.
As described above, the angular velocity ω and the rotation angle θ of the rotor are important for the control of the motor and need to be detected accurately. Thus, although a resolver and encoder having a high accuracy are used as the position detecting sensor 11, there is such a problem that it is expensive.
Japanese Patent Application Laid-open No.2001-204189 A has disclosed a method in which the rotation angle θ of the rotor is detected using the voltage and current of the motor without using any position detecting sensor. However, although this method is capable of detecting the rotation angle θ of the rotor, there is such a problem that the angular velocity ω cannot be detected because this method is based on mainly integration.
As described above, to detect the angular velocity and rotation angle of the rotor accurately, an expensive sensor is needed or if no position detecting sensor is used, the angular velocity cannot be detected although the rotation velocity can be detected.